Power management integrated circuit partitioning with dedicated primary side control winding

ABSTRACT

A switched mode power converter having a feedback mechanism by which a coded train of pulses with well defined integrity is generated on a secondary side of the power converter and transmitted to a dedicated control signal winding on the primary side for decoding and application to regulate the power converter output. The control signal winding enables separation of control signal transmission and power transmission resulting in improved processing of the control signal by a primary side controller. The pulse train is modulated by a secondary side controller, transmitted across a transformer and received by the control signal winding, and supplied to the primary side controller. Coded information is included in the coded pulse train by modulating pulses of the pulse train.

RELATED APPLICATIONS

This patent application claims priority under 35 U.S.C. 119 (e) of theco-pending U.S. Provisional Application Ser. No. 61/799,124, filed Mar.15, 2013, and entitled “New Power Management Integrated CircuitPartitioning With Dedicated Primary Side Control Winding” and theco-pending U.S. Provisional Application Ser. No. 61/793,099, filed Mar.15, 2013, and entitled “New Power Management Integrated CircuitPartitioning”. This application incorporates U.S. ProvisionalApplication Ser. No. 61/799,124 and U.S. Provisional Application Ser.No. 61/793,099 in their entireties by reference.

FIELD OF THE INVENTION

The present invention is generally directed to the field of powerconverters. More specifically, the present invention is directed tocontrolling a power converter.

BACKGROUND OF THE INVENTION

In many applications a power converter is required to provide a voltagewithin a predetermined range formed from a voltage source having adifferent voltage level. Some circuits are subject to uncertain andundesirable functioning and even irreparable damage if supplied powerfalls outside a certain range. More specifically, in some applications,a precise amount of power is required at known times. This is referredto as regulated power supply.

In order to control a power converter to deliver a precise amount ofpower as conditions require, some form of control of the power converteris required. This control can occur on the primary side of an isolationtransformer or the secondary side. A closed loop feedback control systemis a system that monitors some element in the circuit, such as thecircuit output voltage, and its tendency to change, and regulates thatelement at a substantially constant value. Control on the secondary sideof a power converter can use a monitored output voltage as feedbackcontrol, but requires the use of some communication from the secondaryto the primary side of the isolation transformer to control the primaryside switching element. Control on the primary side can readily controlthe primary side switching element, but requires some feedback mechanismfrom the secondary side to the primary side to convey the status of themonitored element. In some applications, an optical coupler circuit, oropto coupler, is used to transmit feedback signals while maintainingelectrical isolation between the primary and secondary sides.

FIG. 1 illustrates a conventional regulated switch mode power converterincluding an optical coupler circuit. The power converter 2 isconfigured as a traditional flyback type converter. The power converter2 includes an isolation transformer 4 having a primary winding P1 and asecondary winding S1. The primary winding P1 is electrically coupled toan input voltage Vin and a driving circuit including a transistor 8, aresistor 12, and a controller 10. A capacitor 28 is coupled across theinput Vin and coupled with the primary winding P1. Input voltage to thecircuit may be unregulated DC voltage derived from an AC supply afterrectification and filtering. The transistor 8 is a fast-switchingdevice, such as a MOSFET, the switching of which is controlled by thefast dynamic controller 10 to maintain a desired output voltage Vout.The controller 10 is coupled to the gate of the transistor 8. As is wellknown, the DC/DC conversion from the primary winding P1 to the secondarywinding S1 is determined by the duty cycle of a pulse width modulation(PWM) switching signal provided to the transistor 8. The secondarywinding voltage is rectified and filtered using the diode 6 and thecapacitor 22. A sensing circuit and a load 14 are coupled in parallel tothe secondary winding S1 via the diode 6. The sensing circuit includes aresistor 16, a resistor 18, and a secondary controller 20. The secondarycontroller 20 senses the output voltage Vout across the load.

In this configuration, the power converter is controlled by drivingcircuitry on the primary side, and the load coupled to the output isisolated from the control. As such, a monitored output voltage used forvoltage regulation is required as feedback from the secondary side tothe control on the primary side. The power converter 2 has a voltageregulating circuit that includes the secondary controller 20 and anoptical coupler circuit. The optical coupler circuit includes twogalvanically isolated components, an optical diode 24 coupled to thesecondary controller 20 and an optical transistor 26 coupled to thecontroller 10. The optical diode 24 provides optical communication withthe optical transistor 26 across the isolation barrier formed by thetransformer 4. The optical coupler circuit in cooperation with thesecondary controller 20 provides feedback to the controller 10. Thecontroller 10 accordingly adjusts the duty cycle of the transistor 8 tocompensate for any variances in an output voltage Vout.

However, the use of an optical coupler circuit in and of itself presentsissues. Firstly, the optical coupler circuit adds extra cost. In someapplications, the optical coupler circuit can add more cost to the powerconverter than the isolation transformer. The optical coupler circuitalso adds to manufacturing and testing costs. Furthermore, theperformance of the optical coupler circuit degrades over time andtherefore introduces another potential point of failure in the overallpower converter. Also, characteristics of the optical coupler circuitmust be accounted for in the overall circuit design. For example, theoptical diode component is non-linear and as such a correlation betweenthe optical diode and the optical transistor must be established. Theoptical coupler circuit also has delays related to the operation of theoptical diode and the optical transistor, and the operation of theoptical diode requires a well defined DC level. As a result, it isgenerally desirable to avoid the use of an optical coupler circuit.

A next generation of feedback control does not use optical controlcircuitry. Instead, the transformer is used to convey real-time feedbacksignaling from the secondary side to the primary side. In such anapplication, the transformer includes an auxiliary winding on theprimary side that is magnetically coupled to the secondary winding. FIG.2 illustrates a conventional regulated power converter including amagnetically coupled feedback circuit. The power converter 32 isconfigured as a traditional flyback type converter. The power converter32 includes an isolation transformer 34 having a primary winding P1 anda secondary winding S1. The primary winding P1 is electrically coupledto an input voltage Vin and a driving circuit including a transistor 44,a resistor 46, and a controller 42. A capacitor 58 is coupled across theinput Vin and coupled with the primary winding P1. Input voltage to thecircuit may be unregulated DC voltage derived from an AC supply afterrectification and filtering. Similar to the power converter in FIG. 1,the transistor 44 is a fast-switching device controlled by the fastdynamic controller 42 to maintain a desired output voltage Vout. Thesecondary winding voltage is rectified and filtered using the diode 36and the capacitor 38, with the output voltage Vout delivered to the load40.

The power converter 32 has a feedback loop that includes a magneticallycoupled feedback circuit coupled to the secondary winding S1 of thetransformer 34 and the controller 42. The magnetically coupled feedbackcircuit includes a diode 48, a capacitor 50, resistors 52 and 54 and anauxiliary winding 56. The auxiliary winding 56 is coupled in parallel tothe series of resistors 52 and 54.

The voltage VA is proportional to the voltage across the auxiliarywinding 56. The voltage VA is provided as a feedback voltage VFB to thecontroller 42. The current through the transistor 44 is also provided asfeedback current IFB to the controller 42. The controller 42 includes areal-time waveform analyzer that analyzes input feedback signals, suchas the feedback voltage VFB and the feedback current IFB.

The auxiliary winding 56 is also magnetically coupled to the secondarywinding S1. When the current through the diode 36 is zero, the voltageacross the secondary winding S1 is equal to the voltage across theauxiliary winding 56. This relationship provides means for communicatingthe output voltage Vout as feedback to the primary side of the circuit.The voltage across the auxiliary winding 56 is measured when it isdetermined that the current through the diode 36 is zero, which providesa measure of the voltage across the secondary winding S1 and thereforethe output voltage Vout.

The feedback voltage VFB when the diode 36 current is zero is determinedand is referred to as the “voltage sense”, and the feedback current IFBwhen the diode 36 current is zero is determined and is referred to asthe “current sense”. The voltage sense and the current sense are inputto the real-time waveform analyzer within the controller 42. FIG. 3illustrates a functional block diagram of a conventional real-timewaveform analyzer 60. Error amplifiers 62 and 64 are acceptors of theregulating means, which in this configuration are the sensed voltage andthe sensed current. The error amplifier compares the input sensedvoltage to a reference voltage and outputs a first difference value. Thefirst difference value is amplified by the gain of the error amplifier62. The error amplifier 64 compares the amplified first difference valueto the sensed current and outputs a second difference value that iseither High or Low. A pulse width modulation (PWM) block 66 isconfigured as a Flip-Flop digital device. The output of the PWM block 66is set according to the switching frequency of the clock 68 and is Resetby the High or Low value input from the error amplifier 64. The variablesignal applied to the Reset pin generates an output signal that is apulse train modulated by the pulse width. A multiple input OR gate 70inputs the clock signal, the pulse train signal, a shutdown signal, anda OVP/UVP/OTP signal, where OVP stands for “over voltage protection”,UVP stands for “undervoltage protection” and OTP stands for “overtemperature protection”. The waveform analyzer is configured to output ahigh voltage value when one of the inputs to the OR gate is high or tooutput a low voltage value when all of the inputs to the OR gate arelow. The high voltage value output from the waveform analyzercorresponds to turning on the transistor 44 in FIG. 2. The low voltagevalue corresponds to turning off the transistor 44. The OR gate alsoenables a high voltage signal output from the PWM block 66 to propagateto the output by monitoring abnormal conditions such as under voltage,over voltage, over temperature, etc. In this manner, the pulse width ofeach pulse can be modified which adjusts the output voltage intoregulation.

In general, control intricacies of the waveform analyzer are alignedwith control argument sampling to achieve overall system functionalperformance. Sampling argument is in the form of current, voltage andimpedance. System functional performance is in the form of pulse widthmodulation (PWM), pulse frequency modulation (PFM) and pulse amplitudemodulation (PAM). The waveform analyzer of FIG. 3 is limited to signalprocessing in DC or real-time switching waveforms. In either case, thefeedback signal received by the waveform analyzer requires some statusintegrity, such as no noise on the DC level, no disturbance on theswitching waveform and to some degree represent a combination of analogand digital representations. The voltage across the auxiliary windingtypically forms a pulse train with frequency corresponding to theswitching frequency of the driving transistor. The voltage across theauxiliary winding when the secondary winding current is zero, whichcorresponds to the diode 36 current equaling zero, corresponds to thefalling edge of the pulse. As such, measuring an accurate voltage valuerequires that the pulse is well defined with sufficient pulse integrityparticularly at the falling edge. Further, the voltage value immediatelyfollowing the rising edge includes ringing due to the leakage impedanceof the transformer. As such, pulse integrity also requires sufficienttime for the voltage value to stabilize following the rising edge.Higher switching frequencies minimize the pulse width and thereforeprovide less time for voltage stabilization. For at least these reasons,providing a pulse with sufficient pulse integrity is often difficult toachieve.

Another type of feedback mechanism uses a second transformer as a gatedrive transformer controlled by a pulse width modulator on the secondaryside of the circuit. FIG. 8 illustrates a conventional regulated powerconverter including a second transformer used as a gate drivetransformer. The power converter 74 is configured as a traditionalflyback type converter. The power converter 74 includes an isolationtransformer 94 having a primary winding P1 and a secondary winding S1.The primary winding P1 is electrically coupled to an input voltage Vinand a driving circuit including a transistor 80, a resistor 78, and adiode 82. A capacitor 84 is coupled across the input Vin and coupledwith the primary winding P1. Input voltage to the circuit may beunregulated DC voltage derived from an AC supply after rectification andfiltering. The driving circuit also includes a diode 96, a controller86, and a gate drive transformer 76 having a primary winding P2 and asecondary winding S2. The diode 96 and the controller 86 are coupled inseries to the secondary winding S2 on the secondary side of the circuit.The primary winding P2 of the transformer 76 is coupled to the gate ofthe transistor 80 via the diode 82. The transistor 80 is afast-switching device controlled by the fast dynamic controller 86 tomaintain a desired output voltage Vout. The voltage across the secondarywinding S1 of the transformer 94 is rectified and filtered using thediode 92 and the capacitor 90, with the output voltage Vout delivered tothe load 80 and to the controller 86. The secondary side controller 86regulates the output voltage Vout by transmitting a driving signal tothe transistor 80 via the gate drive transformer 76.

SUMMARY OF THE INVENTION

Embodiments of a switched mode power converter are directed to afeedback mechanism by which a coded train of pulses with well definedintegrity is generated on a secondary side of the power converter andtransmitted to a dedicated control signal winding on the primary sidefor decoding and application to regulate the power converter output. Thecontrol signal winding enables separation of control signal transmissionand power transmission resulting in improved processing of the controlsignal by a primary side controller. The pulse train is modulated by asecondary side controller, transmitted across a transformer and receivedby the control signal winding, and supplied to the primary sidecontroller. Such a technique allows the transmission of regulation dataover an isolation galvanic barrier without using an optical couplingcircuit. Coded information is included in the coded pulse train bymodulating pulses of the pulse train including, but no limited to, thepulse width, the pulse amplitude, the pulse frequency, or anycombination thereof.

In an aspect, a switching mode power converter is configured for forwardenergy transfer from a primary side to a secondary side and for reverseenergy transfer from the secondary side to the primary side. The powerconverter includes a transformer, a first switch, a first controller, asecond switch, a second controller, a first auxiliary winding and asecond auxiliary winding. The Transformer has a primary winding coupledto an input supply voltage and a secondary winding. The first switch iscoupled in series to the primary winding. The first controller iscoupled to the switch, and the first controller is configured to turnthe first switch ON and OFF. The second switch is coupled in series tothe secondary winding. The second controller is coupled to the secondswitch, and the second controller is configured to turn the secondswitch ON and OFF thereby controlling a negative current through thesecondary winding. The first auxiliary winding is electrically coupledto the first controller and magnetically coupled to the secondarywinding to receive a power component of the reverse energy transfer forproviding power to the first controller. The second auxiliary winding iselectrically coupled to the first controller and magnetically coupled tothe secondary winding to receive a control component of the reverseenergy transfer for providing a feedback signal to regulate a circuitoutput characteristic.

In some embodiments, the second controller is configured to drive thesecond switch to generate negative current through the secondary windingto form a coded control signal, further wherein the transformer isconfigured as a signal transmitter to transmit the coded control signalfrom the secondary side of the transformer to the primary side of thetransformer, wherein the first controller is configured to decode thecoded control signal to identify the control information, generate adriving signal according to the control information, and drive the firstswitch using the driving signal to regulate an output characteristic. Insome embodiments, the coded control signal is a pulse train signalhaving a modulated plurality of voltage pulses. The modulated pluralityof pulses can be modulated by modulating one or more of a pulseamplitude, a pulse frequency, a pulse width, a pulse delay, and a numberof pulse cycles. The second controller is configured to transmit thecoded control signal during an OFF period of the first switch. Theoutput characteristic can be one or more of an output voltage, an outputcurrent, and an output power of the power converter. The power convertercan also include a sensing circuit coupled to the secondary winding andthe second controller, wherein the sensing circuit is configured tosense the output characteristic of the power converter. In someembodiments, the first auxiliary winding has a same polarity as thesecondary winding, and the second auxiliary winding has an oppositepolarity of the secondary winding. In some embodiments, the forwardenergy transfer from the primary side of the transformer to thesecondary side of the transformer does not induce current in the secondauxiliary winding. In some embodiments, first switch is a firsttransistor and the second switch is a second transistor. The powerconverter can also include a diode coupled in parallel to the secondswitch and an output capacitor coupled in series to the diode, whereinthe diode is configured to enable current flow from the secondarywinding to the output capacitor when forward-biased. In thisconfiguration, when the second switch is ON, an alternative current paththrough the second switch is formed between the output capacitor and thesecondary winding of the transformer, further wherein a negativesecondary current flows from the output capacitor to the secondarywinding via the alternative current path. In some embodiments, thesecond auxiliary winding is configured to provide a sampling signal usedfor control by the first controller, the sampling signal is providedwhen the first switch is ON and OFF.

In another aspect, a method of controlling a switching mode powerconverter is disclosed. The method includes configuring a switching modepower converter. The power converter includes a transformer, a firstswitch, a first controller, a second switch, a second controller, afirst auxiliary winding and a second auxiliary winding. The transformerhas a primary winding coupled to an input supply voltage and a secondarywinding. The first switch is coupled in series to the primary winding.The first controller is coupled to the switch. The second switch iscoupled in series to the secondary winding. The second controller iscoupled to the second switch. The first auxiliary winding iselectrically coupled to the first controller and magnetically coupled tothe secondary winding. The second auxiliary winding is electricallycoupled to the first controller and magnetically coupled to thesecondary winding. The first auxiliary winding is dedicated to receive apower transmission for powering the first controller. The secondauxiliary winding is dedicated to receive a coded control signal forcontrolling the first switch. The method also includes driving thesecond switch by the second controller to generate the coded controlsignal, and transmitting the coded control signal from the secondarywinding to the second auxiliary winding. The method also includesdecoding the coded control signal by the first controller to identifycontrol information, and driving the first switch using a driving signalgenerated according to the control information.

In some embodiments, the first auxiliary winding has a same polarity asthe secondary winding, and the second auxiliary winding has an oppositepolarity as the secondary winding. In some embodiments, a forward energytransfer from the primary side of the transformer to the secondary sideof the transformer does not induce current in the second auxiliarywinding. In some embodiments, driving the second switch generatesnegative current through the secondary winding to form the coded controlsignal. In some embodiments, the coded control signal comprises a pulsetrain signal having a modulated plurality of voltage pulses. Themodulated plurality of pulses can be modulated by modulating one or moreof a pulse amplitude, a pulse frequency, a pulse width, a pulse delay,and a number of pulse cycles. The coded control signal is transmittedduring an OFF period of the first switch. The method can also includemeasuring an output characteristic of the secondary side of thetransformer and the coded control signal is generated according to theoutput characteristic. The output characteristic can be one or more ofan output voltage, an output current, and an output power of the powerconverter. In some embodiments, generating the coded control signalincludes generating and applying a driving signal to the second switchto turn the second switch ON and OFF according to a determined patternresulting in a modulated plurality of voltage pulses across thesecondary winding of the transformer. In some embodiments, turning thesecond switch ON enables a negative secondary current through thesecondary winding. In some embodiments, enabling the negative secondarycurrent includes enabling an alternative current path from an outputcapacitor in the output circuit to the secondary winding. In someembodiments, the second auxiliary winding provides a sampling signalused for control by the first controller, the sampling signal isprovided when the first switch is ON and OFF.

BRIEF DESCRIPTION OF THE DRAWINGS

Several example embodiments are described with reference to thedrawings, wherein like components are provided with like referencenumerals. The example embodiments are intended to illustrate, but not tolimit, the invention. The drawings include the following figures:

FIG. 1 illustrates a conventional regulated switch mode power converterincluding an optical coupler circuit.

FIG. 2 illustrates a conventional regulated power converter including amagnetically coupled feedback circuit.

FIG. 3 illustrates a functional block diagram of a conventionalreal-time waveform analyzer.

FIG. 4 illustrates a power converter according to an embodiment.

FIG. 5 illustrates a functional block diagram of a portion of thecontroller for processing the coded voltage pulse train according to anembodiment.

FIG. 6 illustrates a power converter according to another embodiment.

FIG. 7 illustrates the equivalent circuit for the power converter ofFIG. 6 during reverse energy transfer.

FIG. 8 illustrates a conventional regulated power converter including asecond transformer used as a gate drive transformer.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present application are directed to a powerconverter. Those of ordinary skill in the art will realize that thefollowing detailed description of the power converter is illustrativeonly and is not intended to be in any way limiting. Other embodiments ofthe power converter will readily suggest themselves to such skilledpersons having the benefit of this disclosure.

Reference will now be made in detail to implementations of the powerconverter as illustrated in the accompanying drawings. The samereference indicators will be used throughout the drawings and thefollowing detailed description to refer to the same or like parts. Inthe interest of clarity, not all of the routine features of theimplementations described herein are shown and described. It will, ofcourse, be appreciated that in the development of any such actualimplementation, numerous implementation-specific decisions must be madein order to achieve the developer's specific goals, such as compliancewith application and business related constraints, and that thesespecific goals will vary from one implementation to another and from onedeveloper to another. Moreover, it will be appreciated that such adevelopment effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skill in the art having the benefit of this disclosure.

FIG. 4 illustrates a power converter according to an embodiment. Thepower converter 100 is configured to receive an unregulated DC voltagesignal at an input node Vin and to provide a regulated output voltageVout. Input voltage to the circuit may be unregulated DC voltage derivedfrom an AC supply after rectification. The input voltage is typicallyfiltered, such as via capacitor 102.

The power converter 100 is configured as a flyback converter. It isunderstood that the concepts described herein can be applied toalternatively configured switched mode converters including, but notlimed to, a forward converter, a push-pull converter, a half-bridgeconverter, and a full-bridge converter. The power converter 100 includesan isolation transformer 104 having a primary winding P1 and a secondarywinding S1. The primary winding P1 is electrically coupled to the inputvoltage Vin and a driving circuit including a switch 106, a senseresistor 112, and a controller 110. The switch 106 is coupled in serieswith the primary winding P1 of the transformer 104 and the senseresistor 112. The controller 110 is coupled to the switch 106 to turnthe switch ON and OFF.

The power converter 100 further includes output circuitry coupled to thesecondary winding 51 of the transformer 104. The output circuitryincludes a freewheeling rectifier diode 116, a switch 118, a controller120, and an output capacitor 126. The switch 118 is coupled in parallelto the diode 116. An anode of the diode 116 is coupled to a firstterminal of the secondary winding. A cathode of the diode 116 is coupledto a first terminal of the output capacitor 126 and coupled to theoutput node Vout. The output capacitor 126 is coupled to the Vout nodeacross an output load, represented by a resistor 128. The controller 120is coupled to the switch 118 to turn the switch ON and OFF. The outputcircuitry also includes a sensing circuit configured to measure acircuit characteristic to be regulated such as an output voltage, anoutput current, and/or an output power. In this exemplary configurationand succeeding description, the power circuit is described as sensingand regulating the output voltage Vout. In the exemplary configurationof FIG. 4, the sensing circuit includes a resistive voltage dividerincluding the resistors 122 and 124 coupled in parallel to the capacitor126 to measure a voltage across the capacitor 126. It is understood thatan alternative sensing circuit can be used to measure the output voltageVout. In general, the sensing circuit can be configured to use anyconventional technique for determining the value of the regulatedcircuit characteristic.

The switch 106 and the switch 118 are each a suitable switching device.In an exemplary embodiment, the switch 106 and the auxiliary switch 118are each a n-type metal-oxide-semiconductor field-effect transistor(MOSFET) device. Alternatively, any other semiconductor switching deviceknown to a person of skill in the art can be substituted for the switch106 and/or the switch 118. Subsequent description is based on ann-channel MOSFET.

The power converter 100 has a feedback loop that includes a magneticallycoupled feedback circuit coupled to the secondary winding 51 of thetransformer 104. The magnetically coupled feedback circuit includes adiode 108, a capacitor 130, resistors 132 and 134 and an auxiliarywinding 114. The auxiliary winding 114 is coupled in parallel to theseries of resistors 132 and 134. The auxiliary winding 114 is alsomagnetically coupled to the secondary winding S1. When the currentthrough the diode 116 is zero, the voltage across the secondary winding51 is equal to the voltage across the auxiliary winding 114 if the turnsratio is 1:1, or otherwise proportional depending on the turns ratio.This relationship provides means for communicating the voltage acrossthe secondary winding 51 as feedback to the primary side of the circuit.The value of the voltage across the secondary winding 51 is a functionof the secondary current through the secondary winding 51. With thecurrent through the diode 116 equal to zero, the transistor 118 isselectively turned ON and OFF by the controller 120. When the transistor118 is ON, an alternative current path is formed from the chargedcapacitor 126 to the secondary winding 51. The alternative current pathenables negative current flow through the secondary winding 51. In thismanner, the controller 120 generates a driving signal that selectivelyturns the transistor 118 ON and OFF, thereby generating a coded train ofvoltage pulses across the secondary winding 51. The driving signal isconfigured such that the voltage pulses are modulated with codedinformation. In this manner, a coded voltage pulse train is transmittedduring a delay period that corresponds to the switch 106 OFF and thepositive secondary current through the diode 116 having dropped to zero.

In some embodiments, the coded information is the measured outputcircuit characteristic that is to be regulated, such as the outputvoltage Vout. In this case, the controller 120 receives the sensedoutput voltage Vout, and generates a driving signal resulting in amodulated train of voltage pulses across the secondary winding 51 thatis coded to convey the sensed output voltage Vout. In this manner, acoded signal is generated in the form of a coded voltage pulse train,where the DC level of the measured output voltage Vout is coded into thecoded signal. Coded information is included in the coded pulse train bymodulating pulses of the pulse train including, but no limited to, thepulse width, the pulse amplitude, the pulse frequency, or anycombination thereof. For example, the pulse train can be modulated bythe number of pulses over a predetermined time period, or the number ofpulses with different amplitudes over the time period.

The auxiliary winding 114 is magnetically coupled to the secondarywinding S1, and the voltage across the auxiliary winding 114 is equal toor proportional to the voltage across the secondary winding S1 when thecurrent through the diode 116 is zero. As such, the coded voltage pulsetrain is transmitted from across the isolation galvanic barrier via themagnetically coupled auxiliary winding 114 and secondary winding S1.

The coded voltage pulse train across the auxiliary winding 114 ismeasured when the transistor 106 is OFF and the current through thediode 116 equals zero. The voltage VA is proportional to the voltageacross the auxiliary winding 114 and therefore represents the codedvoltage pulse train. The voltage VA is provided as a feedback voltageVFB to the controller 110, wherein the feedback voltage VFB representsthe coded voltage pulse train. In contrast to the conventional powerconverter of FIG. 2 where the feedback voltage VFB is a single pulse perswitching cycle of the main transistor 44, the feedback voltage VFBinput to the controller 110 is a train of pulses per switching cycle ofthe main transistor 106. The train of pulses includes the codedinformation that identifies the measured output voltage Vout, again incontrast to the conventional power converter of FIG. 2 where the singlepulse represents the actual output voltage Vout.

The controller 110 is configured to receive the feedback voltage FB. Thecurrent through the transistor 106 is also provided as feedback currentIFB to the controller 110. The controller 110 includes a real-timewaveform analyzer that analyzes input feedback signals, such as thefeedback voltage VFB and the feedback current IFB. FIG. 5 illustrates afunctional block diagram of a portion of the controller 110 forprocessing the coded voltage pulse train according to an embodiment. Thefeedback voltage VFB input to the controller 110 is received by a pulsetrain acceptor 140. The pulse train acceptor is a digital filterelement, such as a high pass filter, that filters the received codedvoltage pulse train. The filtered signal output from the pulse trainacceptor 140 is input to a signal integrity discriminator 142 where thesignal is decoded and the measured output voltage Vout coded into thecoded voltage pulse train is identified. The signal integritydiscriminator 142 generates and outputs a driving signal thatcorresponds to the identified output voltage Vout. The driving signal isinput to a digital to analog converter 144 and converted to acorresponding DC level.

The DC level output from the converter 144 is input to the waveformanalyzer 146 as the “voltage sense”. The feedback current IFB is inputto the waveform analyzer as the “current sense”. The voltage sense isprovided as a first input to an error amplifier 148. The current senseis provided as a first input to the error amplifier 150. Erroramplifiers 148 and 150 are acceptors of the regulating means, which inthis configuration are the voltage sense and the current sense. Theerror amplifier 148 compares the input voltage sense to a referencevoltage and outputs a first difference value. The first difference valueis amplified by the gain of the error amplifier 148. The error amplifier150 compares the amplified first difference value to the current senseand outputs a second difference value that is either High or Low. Apulse width modulation (PWM) block 152 is configured as a Flip-Flopdigital device. The output of the PWM block 152 is set according to theswitching frequency of the clock 154 and is Reset by the High or Lowvalue input from the error amplifier 150. The variable signal applied tothe Reset pin generates an output signal that is a pulse train modulatedby the pulse width. A multiple input OR gate 156 inputs the clocksignal, the pulse train signal, a shutdown signal, and a OVP/UVP/OTPsignal. The OR gate 156 outputs a high voltage value when one of theinputs to the OR gate is high or to output a low voltage value when allof the inputs to the OR gate are low. The output of the OR gate 156 isamplified by amplifier 158 and output to drive the gate of thetransistor 106 (FIG. 4). The high voltage value output from the OR gate156 corresponds to turning ON the transistor 106 in FIG. 4. The lowvoltage value output from the OR gate 156 corresponds to turning OFF thetransistor 106. The OR gate 156 also enables a high voltage value topropagate to the output by monitoring abnormal conditions such as undervoltage, over voltage, over temperature, etc. In this manner, the pulsewidth of each pulse output from the PWM block 152 can be modified toadjust the output voltage into regulation.

In general, control intricacies of the waveform analyzer are alignedwith control argument sampling to achieve overall system functionalperformance. Sampling argument is in the form of current, voltage andimpedance. System functional performance is in the form of pulse widthmodulation (PWM), pulse frequency modulation (PFM) and pulse amplitudemodulation (PAM). The waveform analyzer shown in FIG. 5 is an exemplaryimplementation. It is understood that alternative circuits andmethodologies can be used to process the DC level output from theconverter 144 and to output a signal for appropriately driving thetransistor 106. It is also understood that the controller 110 can bealternatively configured to process the coded voltage pulse train and togenerate a driving signal for controlling the transistor 106.

In operation, a circuit output characteristic is measured on thesecondary side of a switching mode power converter. In an exemplaryapplication, the circuit output characteristic is the output voltageVout. The secondary side controller generates a driving signal forcontrolling the secondary side transistor while the primary side maintransistor is OFF. The driving signal selectively turns ON and OFF thesecondary side transistor resulting in a coded train of voltage pulsesacross the secondary winding. The measured output voltage Vout is codedinto the coded voltage pulse train. In some embodiments, the codedvoltage pulse train is transmitted from the secondary winding to theauxiliary winding by magnetic coupling between the two windings. Inother embodiments, the coded voltage pulse train is transmitted form thesecondary winding to the auxiliary winding using the parasiticcapacitance of either the transformer or the inherent capacitance of theprinted circuit board across the isolation galvanic barrier, where theprinted circuit board capacitance is due to the formation of thetransformer and corresponding circuitry layout of the power convertercomponents. The coded voltage pulse train is received and decoded by theprimary side controller. The primary side controller identifies themeasured output voltage Vout according to the decoded information andgenerates a driving signal corresponding to the identified outputvoltage Vout. In some embodiments, the driving signal is converted to aDC level that is input as the voltage sense to a waveform analyzer. Thewaveform analyzer uses the input voltage sense to generate a drivingsignal for controlling the primary side main transistor and regulatingthe output voltage Vout.

Although the coded information within the coded voltage pulse train isdescribed above as including information that identifies the outputvoltage Vout, the coded voltage pulse train can be alternativelymodulated to include additional or alternative information. Suchinformation includes, but is not limited to, a simple instruction toturn ON or OFF the primary side main transistor, an indicator of a shortcircuit condition, or an indicator of a voltage out of regulation. Eachtype of information is represented by a separate coding.

In an alternative configuration, a bi-directional switch is used inplace of the diode 116 and the transistor 118. A body diode of thebi-directional switch functions as the freewheeling diode 116. Controlof the bi-directional switch is the same as the transistor 118 to enablea negative secondary current Isec.

In the power converter circuit configuration of FIG. 4, energy istransferred in two directions, a first direction from the primary sideto the secondary side and a second direction from the secondary side tothe primary side. The first direction is a forward energy transfer thatprovides power from the input supply to the output load. The seconddirection is a reverse energy transfer that provides both a powercomponent whereby energy is stored in the capacitor 130 and a controlsignaling component sensed by the primary side controller 110. Energystored in the capacitor 130, supplied by the power component of thereverse energy transfer, along with the power supplied by Vcc, powersthe controller 110. The magnetically coupled auxiliary winding 114enables the reverse energy transfer and the controller 110 discriminatesthe control signaling from the power for operating the controller 110.

In an alternative configuration to the power converter circuit of FIG.4, a second auxiliary winding is added to the primary side circuitry soas to separate the control signaling from the power delivery in thereverse energy transfer direction. FIG. 6 illustrates a power converteraccording to another embodiment. The power converter 200 is configuredto receive an unregulated DC voltage signal at an input node Vin and toprovide a regulated output voltage Vout. Input voltage to the circuitmay be unregulated DC voltage derived from an AC supply afterrectification. The input voltage is typically filtered, such as viacapacitor 202.

The power converter 200 is configured as a flyback converter. It isunderstood that the concepts described herein can be applied toalternatively configured switched mode converters including, but notlimed to, a forward converter, a push-pull converter, a half-bridgeconverter, and a full-bridge converter. The power converter 200 includesan isolation transformer 204 having a primary winding P1 and a secondarywinding S1. The primary winding P1 is electrically coupled to the inputvoltage Vin and a driving circuit including a switch 206, a senseresistor 212, and a controller 210. The switch 206 is coupled in serieswith the primary winding P1 of the transformer 204 and the senseresistor 212. The controller 210 is coupled to the switch 206 to turnthe switch ON and OFF.

The power converter 200 further includes output circuitry coupled to thesecondary winding S1 of the transformer 204. The output circuitryincludes a freewheeling rectifier diode 216, a switch 218, a controller220, and an output capacitor 226. The switch 218 is coupled in parallelto the diode 216. An anode of the diode 216 is coupled to a firstterminal of the secondary winding S1. A cathode of the diode 216 iscoupled to a first terminal of the output capacitor 226 and coupled tothe output node Vout. The output capacitor 226 is coupled to the Voutnode across an output load, represented by a resistor 228. Thecontroller 220 is coupled to the switch 218 to turn the switch ON andOFF. The output circuitry also includes a sensing circuit configured tomeasure a circuit characteristic to be regulated such as an outputvoltage, an output current, and/or an output power. In this exemplaryconfiguration and succeeding description, the power circuit is describedas sensing and regulating the output voltage Vout. In the exemplaryconfiguration of FIG. 6, the sensing circuit includes a resistivevoltage divider including the resistors 222 and 224 coupled in parallelto the capacitor 226 to measure a voltage across the capacitor 226. Itis understood that an alternative sensing circuit can be used to measurethe output voltage Vout. In general, the sensing circuit can beconfigured to use any conventional technique for determining the valueof the regulated circuit characteristic, such as a voltage, current orpower.

The switch 206 and the switch 218 are each a suitable switching device.In an exemplary embodiment, the switch 206 and the auxiliary switch 218are each a n-type metal-oxide-semiconductor field-effect transistor(MOSFET) device. Alternatively, any other semiconductor switching deviceknown to a person of skill in the art can be substituted for the switch206 and/or the switch 218. Subsequent description is based on ann-channel MOSFET.

The power converter 200 has two separate auxiliary windings, a firstauxiliary winding 214 and a second auxiliary winding 236, in the primaryside circuit to enable two paths for transferring energy from thesecondary side to the primary side, referred to as the reverse energytransfer in the second direction. These two separate paths effectivelyseparate the power transfer functionality from the control signalingfunctionality included in the reverse energy transfer. The firstauxiliary winding 214 is part of a power delivery circuit that includesa diode 208 and a capacitor 230. The power delivery circuit is coupledto provide power to the controller 210. The auxiliary winding 214 ismagnetically coupled to the secondary winding 51 and forms a firstreverse energy transfer path that enables power transfer from thesecondary side to the controller 210.

The auxiliary winding 236 is part of a feedback loop for providingcontrol signaling from the secondary side to the controller 210. Theauxiliary winding 236 is magnetically coupled to the secondary winding51. The functionality of the feedback loop in FIG. 6 is similar inoperation to the feedback loop in FIG. 4. When the current through thediode 216 is zero, the voltage across the secondary winding 51 is equalto the voltage across the auxiliary winding 236 if the turns ratio is1:1, or otherwise proportional depending on the turns ratio. Thisrelationship provides means for communicating the voltage across thesecondary winding 51 as feedback to the primary side of the circuit. Thevalue of the voltage across the secondary winding 51 is a function ofthe secondary current through the secondary winding 51. With thetransistor 206 OFF and the current through the diode 216 equal to zero,the transistor 218 is selectively turned ON and OFF by the controller220. When the transistor 218 is ON, an alternative current path isformed from the charged capacitor 226 to the secondary winding 51. Thealternative current path enables negative current flow through thesecondary winding 51. In this manner, the controller 220 generates adriving signal that selectively turns the transistor 218 ON and OFF,thereby generating a coded train of voltage pulses across the secondarywinding S1. The driving signal is configured such that the voltagepulses are modulated with coded information. In this manner, a codedvoltage pulse train is transmitted during a delay period thatcorresponds to the transistor 206 OFF and the positive secondary currentthrough the diode 216 having dropped to zero.

In some embodiments, the coded information is the measured outputcircuit characteristic that is to be regulated, such as the outputvoltage Vout. In this case, the controller 220 receives the sensedoutput voltage Vout, and generates a driving signal resulting in amodulated train of voltage pulses across the secondary winding S1 thatis coded to convey the sensed output voltage Vout. In this manner, acoded signal is generated in the form of a coded voltage pulse train,where the DC level of the measured output voltage Vout is coded into thecoded signal. Coded information is included in the coded pulse train bymodulating pulses of the pulse train including, but no limited to, thepulse width, the pulse amplitude, the pulse frequency, or anycombination thereof. For example, the pulse train can be modulated bythe number of pulses over a predetermined time period, or the number ofpulses with different amplitudes over the time period.

The auxiliary winding 236 is magnetically coupled to the secondarywinding S1, and the voltage across the auxiliary winding 236 is equal toor proportional to the voltage across the secondary winding S1 when thecurrent through the diode 216 is zero. As such, the coded voltage pulsetrain is transmitted from across the isolation galvanic barrier via themagnetically coupled auxiliary winding 236 and secondary winding S1.

The coded voltage pulse train across the auxiliary winding 236 ismeasured when the transistor 206 is OFF and the current through thediode 216 equals zero. The voltage across the auxiliary winding 236 isprovided as a feedback voltage VFB to the controller 210, wherein thefeedback voltage VFB represents the coded voltage pulse train. Thefeedback voltage VFB input to the controller 210 is a train of pulsesper switching cycle of the main transistor 206. The train of pulsesincludes the coded information that identifies the measured outputvoltage Vout, or other output circuit characteristic or abnormality suchas over voltage, under voltage, or over temperature.

The controller 210 is configured to receive the feedback voltage VFB.The current through the transistor 206 is also provided as feedbackcurrent IFB to the controller 210. In general, the controller 210discriminates between regulating voltage or current based on the pulseamplitudes, pulse widths, and/or frequencies of the pulses in the pulsetrain signal. In some embodiments, the controller 210 includes a pulsetrain acceptor, a signal integrity discriminator, an digital to analogconverter, and a real-time waveform analyzer including a pulse widthmodulation (PWM) block. In some embodiments, the pulse train acceptor,the signal integrity discriminator, the digital to analog converter, andthe real-time waveform analyzer perform similar functionality as thepulse train acceptor 140, the signal integrity discriminator 142, thedigital to analog converter 144, and the real-time waveform analyzer146, respectively, in FIG. 5. In some embodiments, the feedback voltageVFB input to the controller 210 is received by the pulse train acceptor.The pulse train acceptor is a digital filter element, such as a highpass filter, that filters the received coded voltage pulse train. Thefiltered signal output from the pulse train acceptor is input to asignal integrity discriminator where the signal is decoded and themeasured output voltage Vout coded into the coded voltage pulse train isidentified. The signal integrity discriminator generates and outputs adriving signal that corresponds to the identified output voltage Vout.The driving signal is input to the digital to analog converter andconverted to a corresponding DC level.

The DC level output from the converter is input to the waveformanalyzer. The feedback current IFB is also input to the waveformanalyzer. The waveform analyzer processes the input feedback signals,such as the feedback voltage VFB and the feedback current IFB, togenerate a driving signal for the transistor 206.

In operation, the power converter 200 functions similarly as the powerconverter 100 except that the control signaling transmitted from thesecondary side is received on the primary side by a winding dedicated toreceive the control signaling, in this case the second auxiliary winding236. The first auxiliary winding 214 is dedicated for power delivery. Assuch, the signal discrimination performed by the controller 210 does notneed to separate the control signaling from the power transmission, asdoes the controller 110 in the power converter 100. The auxiliarywinding 236 is independent of the primary winding S1 and the auxiliarywinding 214, and is recognized only by control signal discriminatingcircuitry within the controller 210. In some embodiments, the auxiliarywinding 236 is dedicated to receive the control signaling while theauxiliary winding 214 is isolated from receiving the control signalingby configuring the two auxiliary windings with opposite polarities.

The polarities of the primary winding P1, the secondary winding S1, thefirst auxiliary winding 214, and the second auxiliary winding 236 areconfigured such that during forward energy transfer where energy isdelivered from the primary side to the secondary side, the primarywinding P1, the secondary winding S1, and the first auxiliary winding214 are in-phase, but the second auxiliary winding 236 is out-of-phasesuch that there is zero voltage across the second auxiliary winding 236during forward energy transmission.

During reverse energy transfer where energy is delivered from thesecondary side to the primary side, the secondary winding S1 and thesecond auxiliary winding 236 are in-phase and the first auxiliarywinding 214 is out-of-phase such that the voltage across the firstauxiliary winding 214 is sufficient to put the diode 208 in theconduction mode, thereby charging the capacitor 230. When the controlsignal is transmitted by the controller 220, such as the pulse trainsignal transmitted during the delay period while the transistor 206 isOFF, the resulting voltage appears across the second auxiliary winding236. In this manner, the control signal is transmitted to the auxiliarywinding 236 and not transmitted to the primary winding P1 because thetransistor 206 is OFF during reverse energy transfer. Since theauxiliary winding 236 is a dedicated winding isolated from the diode 208and the capacitor 230, the control signal supplied as voltage across theauxiliary winding 236 is no longer subject to distortions caused by thediode 208 and the capacitor 230.

Additionally, configuring the auxiliary winding 236 with a differentpolarity than the auxiliary winding 214 enables the controller 210 tosense a positive voltage when the transistor 206 is ON. This providesadditional flexibility for monitoring and thereby controlling circuitconditions when the transistor 206 is ON. When the transistor 206 isOFF, the controller controls the feedback condition through monitoringthe sensed voltage across the auxiliary winding 236, which pointcorresponds to the output voltage Vout during the delay period. When thetransistor 206 is ON, the controller 210 controls another set ofconditions. For example, when the transistor 206 is ON, the voltageacross the auxiliary winding 236 is proportional to the input voltageVin. In this manner, configuring the auxiliary windings 214 and 236 withdifferent polarities provides a wider range of control for thecontroller than the single auxiliary winding configuration as in FIG. 4.The auxiliary winding 236 provides a sampling signal for full cyclecontrol when the transistor 206 either ON or OFF.

FIG. 7 illustrates the equivalent circuit for the power converter 200during reverse energy transfer. In this case, the transistor 206 is OFFand therefore no current flows through primary winding P1. Duringreverse energy transfer from the primary side to the secondary side, thefirst auxiliary winding 214 has an opposite polarity as the secondarywinding S1. With the controller 220 driving the transistor 218, thecontrol signal is transmitted as the reverse energy transfer from thesecondary winding S1 to the second auxiliary winding 236. In thismanner, the second auxiliary winding 236 functions as a dedicatedcontrol winding for receiving the control signal transmitted from thesecondary side.

In some embodiments, the transformer 204 has a single core with fourwindings, the primary winding S1, the secondary winding P1, the firstauxiliary winding 214, and the second auxiliary winding 236. It isunderstood that alternative configurations are contemplated whereby awinding on the primary side is dedicated as a control signal winding.

The present application has been described in terms of specificembodiments incorporating details to facilitate the understanding of theprinciples of construction and operation of the power converter. Many ofthe components shown and described in the various figures can beinterchanged to achieve the results necessary, and this descriptionshould be read to encompass such interchange as well. As such,references herein to specific embodiments and details thereof are notintended to limit the scope of the claims appended hereto. It will beapparent to those skilled in the art that modifications can be made tothe embodiments chosen for illustration without departing from thespirit and scope of the application.

What is claimed is:
 1. A switching mode power converter configured forforward energy transfer from a primary side to a secondary side and forreverse energy transfer from the secondary side to the primary side, thepower converter comprising: a. a transformer having a primary windingcoupled to an input supply voltage and a secondary winding; b. a firstswitch coupled in series to the primary winding; c. a first controllercoupled to the switch, wherein the first controller is configured toturn the first switch ON and OFF; d. a second switch coupled in seriesto the secondary winding; e. a second controller coupled to the secondswitch, wherein the second controller is configured to turn the secondswitch ON and OFF thereby controlling a negative current through thesecondary winding; f. a first auxiliary winding electrically coupled tothe first controller and magnetically coupled to the secondary windingto receive a power component of the reverse energy transfer forproviding power to the first controller; and g. a second auxiliarywinding electrically coupled to the first controller and magneticallycoupled to the secondary winding to receive a control component of thereverse energy transfer for providing a feedback signal to regulate acircuit output characteristic.
 2. The power converter of claim 1 whereinthe second controller is configured to drive the second switch togenerate negative current through the secondary winding to form a codedcontrol signal, further wherein the transformer is configured as asignal transmitter to transmit the coded control signal from thesecondary side of the transformer to the primary side of thetransformer, wherein the first controller is configured to decode thecoded control signal to identify the control information, generate adriving signal according to the control information, and drive the firstswitch using the driving signal to regulate an output characteristic. 3.The power converter of claim 2 wherein the coded control signalcomprises a pulse train signal having a modulated plurality of voltagepulses.
 4. The power converter of claim 3 wherein the modulatedplurality of pulses is modulated by modulating one or more of a pulseamplitude, a pulse frequency, a pulse width, a pulse delay, and a numberof pulse cycles.
 5. The power converter of claim 2 wherein the secondcontroller is configured to transmit the coded control signal during anOFF period of the first switch.
 6. The power converter of claim 2wherein the output characteristic is one or more of an output voltage,an output current, and an output power of the power converter.
 7. Thepower converter of claim 1 wherein the coded control signal is based ona sensed output characteristic.
 8. The power converter of claim 1further comprising a sensing circuit coupled to the secondary windingand the second controller, wherein the sensing circuit is configured tosense the output characteristic of the power converter.
 9. The powerconverter of claim 1 wherein the first auxiliary winding has a samepolarity as the secondary winding, and the second auxiliary winding hasan opposite polarity of the secondary winding.
 10. The power converterof claim 5 wherein the forward energy transfer from the primary side ofthe transformer to the secondary side of the transformer does not inducecurrent in the second auxiliary winding.
 11. The power converter ofclaim 1 wherein the first switch comprises a first transistor and thesecond switch comprises a second transistor.
 12. The power converter ofclaim 1 further comprising a diode coupled in parallel to the secondswitch and an output capacitor coupled in series to the diode, whereinthe diode is configured to enable current flow from the secondarywinding to the output capacitor when forward-biased.
 13. The powerconverter of claim 12 wherein when the second switch is ON, analternative current path through the second switch is formed between theoutput capacitor and the secondary winding of the transformer, furtherwherein a negative secondary current flows from the output capacitor tothe secondary winding via the alternative current path.
 14. The powerconverter of claim 1 wherein the second auxiliary winding is configuredto provide a sampling signal used for control by the first controller,the sampling signal is provided when the first switch is ON and OFF. 15.A method of controlling a switching mode power converter comprising: a.configuring a switching mode power converter comprising a transformerhaving a primary winding coupled to an input supply voltage and asecondary winding, a first switch coupled in series to the primarywinding, a first controller coupled to the switch, a second switchcoupled in series to the secondary winding, a second controller coupledto the second switch, a first auxiliary winding electrically coupled tothe first controller and magnetically coupled to the secondary winding,and a second auxiliary winding electrically coupled to the firstcontroller and magnetically coupled to the secondary winding, whereinthe first auxiliary winding is dedicated to receive a power transmissionfor powering the first controller and the second auxiliary winding isdedicated to receive a coded control signal for controlling the firstswitch; b. driving the second switch by the second controller togenerate the coded control signal; c. transmitting the coded controlsignal from the secondary winding to the second auxiliary winding; d.decoding the coded control signal by the first controller to identifycontrol information; f. driving the first switch using a driving signalgenerated according to the control information.
 16. The method of claim15 wherein the first auxiliary winding has a same polarity as thesecondary winding, and the second auxiliary winding has an oppositepolarity as the secondary winding.
 17. The method of claim 16 wherein aforward energy transfer from the primary side of the transformer to thesecondary side of the transformer does not induce current in the secondauxiliary winding.
 18. The method of claim 15 wherein driving the secondswitch generates negative current through the secondary winding to formthe coded control signal.
 19. The method of claim 15 wherein the codedcontrol signal comprises a pulse train signal having a modulatedplurality of voltage pulses.
 20. The method of claim 19 wherein themodulated plurality of pulses is modulated by modulating one or more ofa pulse amplitude, a pulse frequency, a pulse width, a pulse delay, anda number of pulse cycles.
 21. The method of claim 15 wherein the codedcontrol signal is transmitted during an OFF period of the first switch.22. The method of claim 15 further comprising measuring an outputcharacteristic of the secondary side of the transformer and codedcontrol signal is generated according to the output characteristic. 23.The method of claim 22 wherein the output characteristic is one or moreof an output voltage, an output current, and an output power of thepower converter.
 24. The method of claim 15 wherein generating the codedcontrol signal comprises generating and applying a driving signal to thesecond switch to turn the second switch ON and OFF according to adetermined pattern resulting in a modulated plurality of voltage pulsesacross the secondary winding of the transformer.
 25. The method of claim24 wherein turning the second switch ON enables a negative secondarycurrent through the secondary winding.
 26. The method of claim 25wherein enabling the negative secondary current comprises enabling analternative current path from an output capacitor in the output circuitto the secondary winding.
 27. The method of claim 15 wherein the secondauxiliary winding provides a sampling signal used for control by thefirst controller, the sampling signal is provided when the first switchis ON and OFF.